The present invention relates to an electronically commutatable motor, whose excitation windings are controllable via semiconductor output stages by an electronic control unit with the aid of PWM control signals. A setpoint value can be specified to the control unit, and the control unit emits corresponding PWM control signals to the semiconductor output stages. A motor characteristic curve, from which an assigned nominal operating speed is derivable for the setpoint value is stored in the control unit, and the derived nominal operating speed can be compared to the actual speed of the motor. If a predefinable or predefined speed difference between the nominal operating speed and the actual speed is exceeded, the control unit and/or the semiconductor output stages can be switched off.
A conventional electronically commutatable motor is described in German Published Patent Application 198 04 874. In that case, the PWM control signals are established in their pulse width by the input of the setpoint value. The comparison of the nominal operating speed, which is assigned to the setpoint value, to the actual speed, is used during the continuous running operation for detecting sharp increases of the setpoint value acting from outside, in order to set the pulse width only gradually to the new value. Since the motor characteristic curve changes as a function of the motor load and the setpoint value, it requires a considerable expenditure of memory in the control unit to ascertain the allocated nominal operating speed.
Storage of the characteristic-curve data of a motor in a memory of the control unit and use of the characteristic-curve data for deriving an operating value is discussed to some extent in the U.S. Pat. No. 5,901,286 and European Published Patent Application No. 0 886 057. In these references, a characteristics field having a plurality of value pairs is used, from which the desired nominal operating value can be derived by interpolation onto a third coordinate. However, this requires a considerable expenditure of memory, particularly when the load of the motor changes.
The object of the present invention is to provide a motor of the type mentioned at the outset with simple data in the control unit, which, with minimal expenditure, for a predefined load, significantly simplifies the derivation of the nominal operating speed corresponding to a predefined setpoint value.
According to the present invention, this objective is achieved by storing the motor characteristic curve only as a three-dimensional characteristics field having four corner points, which, through coordination with the smallest pulse width and the limiting values of the supply voltage, as well as with the largest pulse width and the limiting values of the supply voltage, are determined by the nominal operating speeds assigned in each case. The nominal operating speed for the comparison to the actual speed is derivable as a function of the existing supply voltage, the predefined setpoint value and the stored coordinate values of the characteristics field.
In this context, advantage is taken of the fact that in many cases, the motor is always loaded with the same consumer, such as in the case of a fan drive. The four coordinate values of the characteristics field take into account not only the pulse widths of the PWM control signals corresponding to the predefinable setpoint values, but also the fluctuations of the supply voltage, and define a characteristics field which allows a clear and simple derivation, i.e. calculation of the assigned nominal operating speed, for the supply voltage present in each case and the control conditions, the connecting lines of the corner points of the characteristics field giving the stipulations for a grid, and thus facilitating the derivation of intermediate values in the coordinate directions for the supply voltage (e.g. x-coordinate) and the pulse widths (e.g. z-direction), and leading to the sought nominal-operating speed (in the y-direction).
Depending upon the use of the motor, according to a further embodiment, the four corner points of the characteristics field may be determined for a predefined motor load. The motor can then be designed in a simple manner for a different load, i.e. consumer.
In this context, according to one refinement of the present invention, the comparison between the nominal operating speed and the actual speed is able to be carried out continually during the continuous running of the motor or repeated at time intervals.
The setpoint value may be specifiable manually in a simple manner using a potentiometer, the control unit being able to be supplied with a variable setting signal which is used for the emission of allocated PWM control signals for the semiconductor output stages. In addition, using this setting signal, the allocated nominal operating speed may be derived on the basis of the stored motor characteristic curve and utilized for the comparison with the actual speed of the motor arising. The actual speed of the motor may be detected in various different ways.
For the comparison of the nominal operating speed and the actual speed, the control unit is coupled to a comparator unit which may be integrated into the control unit.
So that the overload protection does not react to short interference pulses of the actual-speed measurement, one embodiment of the present invention provides for the control unit and/or the semiconductor output stages to be switched off in a time-delayed manner.
If a run-up phase precedes the continuous operation of the motor,-then the overload protection may be designed so that the comparison of the nominal operating speed and the actual speed is first able to be initiated and carried out after a run-up phase of a predefined duration has expired, so that an inadvertent shut-down does not occur during this operating phase. The run-up phase may be preset by the control unit, and the amplitude of the pulses and the pulse width of the PWM control signals, as well as their commutation frequency may be used as parameters. The run-up phase of the motor is able to be initiated with the switch-on of the control unit and/or the semiconductor output stages, and/or the input of a setpoint value for the control unit.